Answer
$[PH_3]$= 0.0432 mol/L
$[BCl_3]$= 0.0432 mol/L
Work Step by Step
Kc = $\frac{products}{reactants}$ = $\frac{[PH_3][BCl_3]}{1}$
We denote the reactants as (1) because the reactant $PH_3$$BCl_3$$(s)$ is a solid. Solids and liquids are not included in equilibrium constants.
Step 1:
$Kc = [PH_3][BCl_3]$
Therefore:
$1.87\times 10^{-3} = [PH_3][BCl_3]$
Step 2:
In the reaction the mole ratio for the products $[PH_3][BCl_3]$ is one to one.
Therefore:
We can denote $[PH_3][BCl_3]$ as $[x][x]$.
Step 3:
$[x][x]$ = $x^{2}$
So: $1.87\times 10^{-3} = x^{2}$
Using algebraic laws for exponents, we see that by squaring both sides of the equation the answer is:
$x = 0.0432$ $mol/L$
Rounded to three significant digits.
